(1) Field of the Invention
The present invention relates to a sealing element, hermetic container and its sealing method for keeping contamination-averse items clean. More detailedly, the present invention relates to a sealing element, hermetic container and its sealing method to be used for accommodation and shipment of contamination-averse precision substrates such as semiconductor wafers, masking glass plates, liquid crystal cells, recording media, etc., and to be used for positioning precision substrates to processing machines which shape and process the substrates and for transportation and storage of them between processing machines.
(2) Description of the Prior Art
There are a variety of types of plastic storage containers for accommodating contamination-averse items. As one example, hermetic containers used for production of semiconductor parts can be mentioned. Large-diametric development(e.g., 300 mm or 400 mm or greater) of precision substrates used for fabrication of semiconductor parts, such as semiconductor wafers, masking glass substrates etc., has been discussed and demanded, aiming at reduction in cost by improving the production yield of semiconductor substrates, as price competition of semiconductor devices has become more severe. At the same time, semiconductor circuits have become more and more miniaturized. For example, the design rule (the minimum line width in processing) of DRAMs (dynamic random access memory) has been being shifted from 0.25 μm to 0.18 μm or less. Needless to say at the factories where semiconductor substrates are processed, there have been demands for high cleanness of the containers which are used to store semiconductor substrates when transported.
In order to meet such demands, a method, called ‘SMIF’ in abbreviation, has been proposed, which assures highly clean surroundings within limited, local spaces required for fabrication of semiconductor substrates and hermetically keeps a plurality of semiconductor substrates clean in a hermetic container so as to transport the hermetic container between several clean environmental spaces. To realize this method, development of hermetic containers which can be automatically conveyed without letting the precision substrates therein be contaminated and may allow itself direct access to the processing equipment is in progress.
As partly shown in FIGS. 1 and 2, a conventional hermetic container is comprised of a container body 1 having an opening on the front, a door element 11 detachably fitted to open and close the open front 9 of the container body 1 and a sealing element 20 interposed between the container body 1 and door element 11 for keeping the inside clean. This sealing element 20 may be a squeeze type made up of an O-ring having a circular section or a type shown in FIGS. 1 and 2.
The sealing element 20 of this type is formed of an endless molding of a material selected from various types of rubbers or elastomers. This sealing element 20, as shown in the same figures, is configured of a deformable endless portion 21 fitted into a fit-holding groove 16 formed on the outer peripheral side of door element 11, a recessed portion 25 of an approximately rectangular section, formed on the underside closer to the outer periphery of the endless portion 21 to create a clearance relative to door element 11, and a deformable rib 26 projected perpendicularly at the obverse surface on the outer peripheral side of endless portion 21 so as to come into contact with the inner periphery of the open front of container body 1.
When door element 11 is fitted to the open front 9 of container body 1 with a multiple number of precision substrates aligned and stored therein, this sealing element 20, especially part of endless portion 21 and rib 26 having a pin-like cross section, deform while recessed portion 25 adjusts the compressive force, to thereby totally seal the open front of container body 1.
Since, in the conventional hermetic container, sealing element 20 is formed of a mere molding of a rubber or elastomer, which cannot afford the necessary dimensional accuracy and the necessary deformability, it becomes markedly difficult, as the size of the opening of container body 1 becomes greater, to secure hermetic confinement by making sealing element 20 into uniform contact with the inner periphery of the open front. Particularly, the sealing element reveals marked deterioration of its sealing ability at the corners of open front 9 of container body 1, due to pressures in two different directions and its dimensional errors.
The above problem can be solved by making sealing element 20 undergo a greater amount of squeezing. However, upon standardization of the specifications of the device for opening and closing door element 11, the pressing force allowed to act on the opening and closing device of door element 11 is limited by an upper boundary, in order to protect the device and the hermetic container. Therefore, if the repulsive force of sealing element 20 exceeds the upper boundary, the repulsive force of sealing element 20 will apply a load greater than necessary on the opening and closing device of door element 11, causing a risk of interrupting the operation of door element 11. The sealing element 20 of the type shown in FIG. 1 presents its sealing function through rib 26, but this rib 26 can bend either inwards or outwards hence the direction of deformation is largely unknown, so that if the rib 26 flexes in reverse to the expected direction, the sealability may become uneven at that portion.
Further, since in local environments, the precision substrates should be loaded from the hermetic container for each processing step and be unloaded after each process, door element 11 of the hermetic container will be repeatedly opened and closed. Therefore, it frequently happens that sealing element 20 may displace from the proper position due to repeated opening and closing actions and be squeezed intensively between container body 1 and fit-holding groove 16 of door element 11 to be deformed greater than necessary. There is also another problem where expected sealability cannot be obtained or resin powder or particles may arise due to local rubbing of sealing element 20 when open front 9 of container body 1 is closed by door element 11, contaminating the precision substrates therein.
Moreover, when the hermetic container is washed in order to keep it clean, the conventional sealing element 20 has been washed while remaining fitted on container body 1 or door element 11. However, when a type of sealing element shown in FIG. 1 is used, not only is the sealing element not washed thoroughly because recessed portion 25 cannot be cleaned well, but also there is a risk that water droplets W might be left over. Therefore, a sealing element of this type is poor in cleansability and drainage, and takes a very long time to dry.